As a cardinal, Ghislieri gained a reputation for putting orthodoxy before personalities, prosecuting eight French bishops for heresy. He also stood firm against nepotism, rebuking his predecessor Pope Pius IV to his face when he wanted to make a 13-year old member of his family a cardinal and subsidise a nephew from the papal treasury.[5]

As prior of more than one Dominican priory during a time of great moral laxity, he insisted on discipline, and, in accordance with his own wishes, was appointed inquisitor at Como. As his reformist zeal provoked resentment, he was compelled to return to Rome in 1550, where, after having been employed in several inquisitorial missions, he was elected to the commissariat of the Holy Office. Pope Paul IV (1555–59), who, as Cardinal Carafa, had shown him special favor, conferred upon him the bishopric of Sutri and Nepi, the cardinalate with the title of Alessandrino, and the unique honor of the supreme inquisitorship. Under Pope Pius IV (1559–65) he became bishop of Mondovi in Piedmont, but his opposition to that pontiff procured his dismissal from the palace and the abridgment of his authority as inquisitor.[8]

Before Michele Ghislieri could return to his episcopate, Pope Pius IV died. On 8 January 1566, Ghislieri was elected to the papal throne as Pope Pius V.[7] He was crowned ten days later, on his 62nd birthday by the protodeacon.

Aware of the necessity of restoring discipline and morality at Rome to ensure success without, he at once proceeded to reduce the cost of the papal court after the manner of the Dominican Order to which he belonged, compel residence among the clergy, regulate inns, expel prostitutes,[5] and assert the importance of the ceremonial in general and the liturgy of the Mass in particular. In his wider policy, which was characterised throughout by an effective stringency, the maintenance and increase of the efficacy of the Inquisition and the enforcement of the canons and decrees of the Council of Trent had precedence over other considerations.

Accordingly, in order to implement a decision of that council, he standardised the Holy Mass by promulgating the 1570 edition of the Roman Missal. Pius V made this Missal mandatory throughout the Latin rite of the Catholic Church, except where a Mass liturgy dating from before 1370 AD was in use.[9][10] This form of the Mass remained essentially unchanged for 400 years until Pope Paul VI's revision of the Roman Missal in 1969–70, after which it has become widely known as the Tridentine Mass;[11] use of the last pre-1969 edition of the Missal, that by Pope John XXIII in 1962, is permitted without limitation for private celebration of the Mass and, since July 2007, is allowed also for public use, as laid down in the motu proprio Summorum Pontificum of Pope Benedict XVI. Some continue to use even earlier editions, but without authorisation.

Pius V recognized attacks on papal supremacy in the Catholic Church and was desirous of limiting their advancement. In France, where his influence was stronger, he took several measures to oppose the Protestant Huguenots. He directed the dismissal of Cardinal Odet de Coligny[13] and seven bishops, nullified the royal edict tolerating the extramural services of the Reformers, introduced the Roman catechism, restored papal discipline, and strenuously opposed all compromise with the Huguenot nobility.

In the list of more important bulls he issued, the famous bull "In Coena Domini" (1568) takes a leading place; but amongst others throwing light on Pope Pius V's character and policy there may be mentioned his prohibition of quaestuary (February 1567 and January 1570); the condemnation of Michael Baius, the heretical Professor of Leuven (1567); the reform of the breviary (July 1568); the denunciation of homosexual behaviour by the clergy;[14] (August 1568)[citation needed]; the banishment of the Jews from the ecclesiastical dominions except Rome and Ancona (1569);[15] the injunction of the use of the reformed missal (July 1570); the confirmation of the privileges of the Society of Crusaders for the protection of the Inquisition (October 1570); the suppression of the Fratres Humiliati for profligacy (February 1571); the approbation of the new office of the Blessed Virgin (March 1571); the enforcement of the daily recitation of the Canonical Hours (September 1571); and the purchase of assistance against the Turks by offers of plenary pardon (March 1572).

Katherine Rinne says, in Waters of Rome,[16] that Pius V also ordered the construction of public works to improve the water supply and sewer system of Rome.

His response to the Queen Elizabeth I of England assuming governance of the Church of England included support of the imprisoned Mary, Queen of Scots, and her supporters in their attempts to take over England"ex turpissima muliebris libidinis servitute""[from the sordid libidinous slavery to women]". A brief English Catholic uprising, the Rising of the North, had just failed. Pius then issued a bull, Regnans in Excelsis, dated 27 April 1570, that declared Elizabeth I a heretic and released her subjects from their allegiance to her.[17] In response, Elizabeth, who had thus far tolerated Catholic worship in private, now actively started persecuting them for treason.

Pius V is often credited with the origin of the Pope's white garments, supposedly because after his election Pius continued to wear his white Dominicanhabit. However, many of his predecessors also wore white with a red mozzetta, as can be seen on many paintings where neither they nor Pius is wearing a cassock, but thin, wide, white garments.

An article by Agostino Paravicini Bagliani on L'Osservatore Romano of 31 August 2013 states that the earliest document that speaks explicitly of the Pope wearing white is the Ordo XIII, a book of ceremonies compiled in about 1274 under Pope Gregory X. From that date on, the books of ceremonies speak ever more explicitly of the Pope as wearing a red mantle, mozzetta, camauro and shoes, and a white cassock and stockings.[19][20]

Pius V died on 1 May 1572 of what is believed to be cancer. He was buried in the chapel of S. Andrea which was close to the tomb of Pope Pius III, in the Vatican. Despite that his will requested he be buried in Bosco, Pope Sixtus V built a monument in the chapel of SS. Sacramento in the Liberian basilica. His remains were transferred there on 9 January 1588.

In the following year, 1713, his feast day was inserted in the General Roman Calendar, for celebration on 5 May, with the rank of "Double", the equivalent of "Third-Class Feast" in the General Roman Calendar of 1960, and of its present rank of "Memorial".[24] In 1969 the celebration was moved to 30 April, the day before the anniversary of his death (1 May).

Portrait of Pius V by Pierre Le Gros

The front of his tomb has a lid of gilded bronze which shows a likeness of the dead pope. Most of the time this is left open to allow the veneration of the saint's remains.

Pius V also helped financially in the construction of Valletta, Malta's capital city, by sending his military engineer Francesco Laparelli to design the fortification walls.